📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/css-tree/lib/utils/clone.js

@@ -0,0 +1,21 @@
+var List = require('../common/List');
+
+module.exports = function clone(node) {
+    var result = {};
+
+    for (var key in node) {
+        var value = node[key];
+
+        if (value) {
+            if (Array.isArray(value) || value instanceof List) {
+                value = value.map(clone);
+            } else if (value.constructor === Object) {
+                value = clone(value);
+            }
+        }
+
+        result[key] = value;
+    }
+
+    return result;
+};

frontend/node_modules/css-tree/lib/utils/createCustomError.js

@@ -0,0 +1,17 @@
+module.exports = function createCustomError(name, message) {
+    // use Object.create(), because some VMs prevent setting line/column otherwise
+    // (iOS Safari 10 even throws an exception)
+    var error = Object.create(SyntaxError.prototype);
+    var errorStack = new Error();
+
+    error.name = name;
+    error.message = message;
+
+    Object.defineProperty(error, 'stack', {
+        get: function() {
+            return (errorStack.stack || '').replace(/^(.+\n){1,3}/, name + ': ' + message + '\n');
+        }
+    });
+
+    return error;
+};

frontend/node_modules/css-tree/lib/utils/names.js

@@ -0,0 +1,104 @@
+var hasOwnProperty = Object.prototype.hasOwnProperty;
+var keywords = Object.create(null);
+var properties = Object.create(null);
+var HYPHENMINUS = 45; // '-'.charCodeAt()
+
+function isCustomProperty(str, offset) {
+    offset = offset || 0;
+
+    return str.length - offset >= 2 &&
+           str.charCodeAt(offset) === HYPHENMINUS &&
+           str.charCodeAt(offset + 1) === HYPHENMINUS;
+}
+
+function getVendorPrefix(str, offset) {
+    offset = offset || 0;
+
+    // verdor prefix should be at least 3 chars length
+    if (str.length - offset >= 3) {
+        // vendor prefix starts with hyper minus following non-hyper minus
+        if (str.charCodeAt(offset) === HYPHENMINUS &&
+            str.charCodeAt(offset + 1) !== HYPHENMINUS) {
+            // vendor prefix should contain a hyper minus at the ending
+            var secondDashIndex = str.indexOf('-', offset + 2);
+
+            if (secondDashIndex !== -1) {
+                return str.substring(offset, secondDashIndex + 1);
+            }
+        }
+    }
+
+    return '';
+}
+
+function getKeywordDescriptor(keyword) {
+    if (hasOwnProperty.call(keywords, keyword)) {
+        return keywords[keyword];
+    }
+
+    var name = keyword.toLowerCase();
+
+    if (hasOwnProperty.call(keywords, name)) {
+        return keywords[keyword] = keywords[name];
+    }
+
+    var custom = isCustomProperty(name, 0);
+    var vendor = !custom ? getVendorPrefix(name, 0) : '';
+
+    return keywords[keyword] = Object.freeze({
+        basename: name.substr(vendor.length),
+        name: name,
+        vendor: vendor,
+        prefix: vendor,
+        custom: custom
+    });
+}
+
+function getPropertyDescriptor(property) {
+    if (hasOwnProperty.call(properties, property)) {
+        return properties[property];
+    }
+
+    var name = property;
+    var hack = property[0];
+
+    if (hack === '/') {
+        hack = property[1] === '/' ? '//' : '/';
+    } else if (hack !== '_' &&
+               hack !== '*' &&
+               hack !== '$' &&
+               hack !== '#' &&
+               hack !== '+' &&
+               hack !== '&') {
+        hack = '';
+    }
+
+    var custom = isCustomProperty(name, hack.length);
+
+    // re-use result when possible (the same as for lower case)
+    if (!custom) {
+        name = name.toLowerCase();
+        if (hasOwnProperty.call(properties, name)) {
+            return properties[property] = properties[name];
+        }
+    }
+
+    var vendor = !custom ? getVendorPrefix(name, hack.length) : '';
+    var prefix = name.substr(0, hack.length + vendor.length);
+
+    return properties[property] = Object.freeze({
+        basename: name.substr(prefix.length),
+        name: name.substr(hack.length),
+        hack: hack,
+        vendor: vendor,
+        prefix: prefix,
+        custom: custom
+    });
+}
+
+module.exports = {
+    keyword: getKeywordDescriptor,
+    property: getPropertyDescriptor,
+    isCustomProperty: isCustomProperty,
+    vendorPrefix: getVendorPrefix
+};